The treatment of tumor can be approached by several modes of therapy, including surgery, radiation, chemotherapy, and a combination of any of these treatments. Among them, chemotherapy is indispensable for inoperable or metastatic forms of cancer. Considering the diversity of tumors in terms of cell type, morphology, growth rate, and other cellular characteristics, the U.S. National Cancer Institute (NCI) has developed a “disease-oriented” approach to anti-tumor activity screening. Boyd, M. R. (1989) In Principle of Practice of Oncology Devita, J. T., Hellman, S., and Rosenberg, S. A. (Eds.) Vol. 3, PPO Update, No. 10. This in vitro screening system is based on human tumor cell line panels consisting of approximately 60 cell lines of major human tumors (e.g., leukemia, lung cancer, colon cancer, CNS cancer, melanoma, ovarian cancer, renal cancer, prostate cancer, and breast cancer), and serves as a tool for identifying compounds that possess anti-tumor activities. Among the compounds, modified nucleobases are of particular interest.